Aqueous coating compositions of a resinous thermoplastic coating material (clearcoat) such as thermoplastic, (meth)acrylic or (meth)acrylic-styrene copolymer in the form of emulsions are well known in the printing industry and presently are being used to coat inked and uninked layers during wet-trap, off-line dry-trap, gravure, offset, silk-screen, flexography and related printing or coating processes using an aqueous coating composition.
In one aspect of the above-referenced printing processes, an ink layer is first put down on a substrate in the form of paper, cloth, fiberboard, corrugated box, etc. and depending upon the process, the ink layer is first allowed to dry before it is coated, or is coated wet. In other methods according to the present invention, the coating may simply be placed onto an uninked or ink-free substrate. The aqueous coating serves to provide certain film characteristics including gloss, mar resistance, oil and water resistance, MVTR, and protection of the inked, uninked or related surface, adhesion and other characteristics. These film characteristics are generally determined by the weight of the coating applied and the amount or percent of solids used in the coating composition.
The prior art materials used as coatings in combination with the current print coating techniques are grossly limited in the solid contents that may be uniformly deposited onto a substrate from a coating composition and the degree of gloss value that a coating may obtain. In addition, as presently employed, the formulation of one aqueous coating may only be used in one or perhaps two processes; it is virtually impossible using the present methods without the present invention to provide one formulation which may be readily adapted for use in wet-trap, off-line dry-trap, gravure, off-set, silk-screen, flexography and other printing processes.
In wet-trap in-line printing processes an ink coating (usually a hydrophobic ink) is first deposited onto paper, fiberboard, cardboard, corrugated paper or similar material, as a wet ink and then an aqueous coating is deposited onto the wet ink layer such that the ink is "trapped" under the aqueous coating to provide adequate film characteristics. In dry-trap off-line printing processes the ink is first dried before an aqueous coating is deposited onto the ink layer.
Gravure and flexography printing processes employ plates or etched cylinders (generally containing inverted pyramids) to deposit the ink layer (usually a water-based or solvent-based ink) which is generally dried before being coated by an aqueous coating. The result is a smooth finish without screen or dot pattern. In these applications, it is critical to have adequate mechanical transfer and flow characterisics to obtain adequate surface tension and favorable film characteristics after deposition.
In offset printing processes, the image to be reproduced is copied photographically upon a metal plate with a solution containing water to prevent the ink from adhering to the non-image area. When placed upon the appropriate cylinder of an offset press, the metal plate is inked in the image area only and makes an imprint of the image on a rubber-covered cylinder, which in turn, prints upon sheets of paper which are automatically fed into the machine. After the image has been deposited onto the paper, it may be coated using an aqueous coating in order to enhance the physical characteristics of the ink surface. Newer techniques in offet utilize waterless plates which keep the ink from adhering to the non-image area without the use of water, alcohol or fountain solution.
Silk-screen is a process employing a stencil to print a flat color design through a piece of silk or other fine cloth on which all parts of the design not to be printed have been stamped out by an impermeable substance.
The viscosity and consequently, the flow characteristics and mechanical transfer of an aqueous coating composition as used in printing processes, are directly influenced by the chemistry of the formulation, in particular, the percentage of solids that are present in the composition. In general, as the amount of solids in an aqueous coating composition increases, the mechanical transfer of the coating generally suffers, because the coating composition becomes too viscous to be efficiently deposited using the techniques presently available in the art. Often, the viscosity of an aqueous composition is the limiting factor in determining the transfer and the degree of usefulness of the coating composition. In general, upon application of an aqueous coating composition onto an inked, uninked or related layer, acceptable mechanical transfer will provide for a coating evidencing acceptable flexibility, durability, film-thickness and gloss, among other favorable film characteristics. In compositions which are too viscous, i.e., have poor flow characteristics and thus evidence inadequate mechanical transfer, the tendency is to produce a coating which evidences a "ribbing" or an uneven deposition of the coating. Inconsistency generally results from a coating having high viscosity.
The standard measure of aqueous coating viscosity in the printing industry is generally determined using a Zahn cup or equivalent. Zahn cups are identified with numbers representing the size of flow holes in cups. For example, the #2 cup is designed with a smaller hole than the #3 cup. To determine viscosity, a cup is chosen and then dipped into the aqueous coating composition until it is filled to the top. The composition will exit the cup from the hole depending upon the size of the hole and the viscosity of the composition measured. The composition stream leaving the cup is then timed with a stopwatch until the cup empties. The time that the composition takes to completely exit the Zahn cup hole in seconds represents the composition's viscosity. The viscosities of compositions may be compared directly based upon the equipment and the mechanical application used. Often the selection of a type of Zahn cup design used is based on the type of printing method utilized.
It is commonly known in the trade, for example, that the viscosity values (measured using a Zahn Drip Cup or equivalent measuring device) necessary for effective mechanical transfer for all printing methods will vary, based upon the mechanics of that printing process. For example, in the case of gravure printing processes, the viscosity for an aqueous coating useful in this process ranges from about 17 to about 28 seconds measured with a #2 Drip Cup. Silk screen printing requires a viscosity range of about 12 to about 23 seconds (#2 Drip Cup). In the case of flexography printing, the viscosity of the aqueous coating ranges from about 20 to about 60 seconds (=2 Drip Cup). In the case of offset printing, the viscosity of the aqueous coating ranges from about 15 to 30 seconds (#3 Drip Cup). One of ordinary skill will understand these values to represent exemplary useful ranges for practicing the present invention. The actual ranges may vary depending on the equipment and application used.
Under the present practice in the industry, the method employed for changing the viscosity of an aqueous coating formulation once it reaches the printing plant is to change the chemistry of the formulation, i.e., adjust the viscosity of the formulation by adding resinous material to increase viscosity or alternatively, by adding solvent to decrease viscosity. This is a time consuming and inefficient practice, especially where there is a need to use an aqueous coating in more than one type of printing process. To avoid this problem, there presently is a need to have several formulations of aqueous coating on hand, in order to accommodate the varying mechanical transfer requirements of the various printing processes. One aqueous coating formulation will simply not suffice.
In the present practice, the transfer of the aqueous coating composition is limited by the viscosity, which is affected by the amount of solids contained in the composition. As one increases the amount of solids, the viscosity of the aqueous coating also increases. It is generally recognized that as the amount of resin in the aqueous coating increases, the gloss, durability, film-thickness and related coating characteristics may tend to increase. Present coatings, however, are limited in the amount of solids that can be used without so dramatically increasing the viscosity of the coating formulations that they cannot be used in traditional printing processes. The present invention seeks to address this limitation to produce coatings having extremely high gloss, durability and film-thicknesses heretofore unknown in the printing industry using coating compositions which can be easily adapted for use in virtually all printing processes.
One of the major problems facing the printing industry is the need for using large amounts of volatile organic compounds or VOC's in aqueous coating compositions. Although a major component of an aqueous coating composition is water, in a majority of cases, in order to produce compositions containing high solid content, VOC's are added to the aqueous composition to lower the viscosity of high solids content compositions. At present, it is often not feasible to produce high solids content aqueous compositions without adding substantial quantities (greater than about 5% by weight) of at least one VOC, such as ethanol, isopropanol, a ketone, ether or the like. The addition of the VOC in present aqueous compositions is known to compatibilize the solids in the composition, thus producing a less viscous product than is produced without the VOC. Even with the VOC, however, the amount of solids that may be added to a composition is quite limited; the result is an aqueous coating composition which cannot produce the extremely favorable coating characteristics (especially high gloss values in combination with mar resistance, durability and flexibility) which are desired in today's market and which are produced using the method of the present invention.
The present invention may be adapted to provide extremely favorable coating characteristics, including high gloss value, increased film integrity and enhanced mar resistance without having to resort to the inclusion of substantial quantities of VOC's (which is the present practice). Thus, it is finally possible to formulate a single coating composition which will exhibit favorable mechanical transfer during coating and favorable film characteristics after deposition. This is an unexpected result. Thus, by utilizing the present invention, a single aqueous composition containing low VOC's or even an absence of VOC's can be generally adapted to a number of printing methods to provide exceptionally favorable coating and mechanical transfer.
In the food industry, paperboard having a moisture barrier coating has recently been used to replace polyboard (for use as food trays and related plastic food packaging material) for providing MVTR and oil and water resistance in storing food. In its present form, a moisture barrier coating (in preferred embodiments also incorporating oil and water resistance) is coated onto the surface of the paperboard so as to ultimately create a surface which can influence the moisture vapor transition rate and lower it to a level which is compatible with the storage of food, especially meat, poultry and other perishable items. Presently however, in order to create a coating thick enough or dense enough to materially impact the moisture vapor transition rate, an aqueous coating solution must be applied at least two or three times on a paperboard surface and subsequently dried. This has created great inefficiency in producing food packaging material and a clear need in the art exists for a process which can produce an adequate barrier coating on paperboard in only one coat. The method according to the present invention may be used to provide a barrier coating on paperboard in only one application, unlike the prior art methods.